The invention pertains to a solar cell unit with a removable top layer.
Solar cell units as a rule comprise a photovoltaic (PV) layer composed of a semiconductor material which is provided between a front electrode (at the front of the unit, i.e. on the side of the incident light) and a back electrode (at the back of the unit). The front electrode is transparent, enabling incident light to reach the semiconductor material, where the incident radiation is converted into electric energy. In this way light can be used to generate electric power, which offers an interesting alternative to, say, fossil fuels or nuclear power.
The top side of the solar cell unit, i.e. the side of the front electrode, is protected from the effects of its surroundings by a transparent top layer which comprises, e.g., sheet of glass or a plastics based sheet or layer. When the solar cell unit is in use, its top layer will in due course get dirty, for instance because of soot, sand, and other dirt from the surrounding area being deposited thereon. As a result the transparency of the top layer will decrease, causing a reduction in the yield of the solar cell unit. When the top layer is made of glass, it can be cleaned in principle, but this is a labour intensive process, not least because solar cell units are often to be found on roofs or in otherwise poorly accessible places. Plastics based top layers generally are more fragile than top layers based on a glass sheet. This means that, on the one hand, the risk of damage to the top layer from its surroundings, e.g., because of sand, is greater still, while, on the other hand, cleaning of the top layer is even more problematic. In other applications, such as portable applications into which solar cells have been integrated, it is conceivable that in due course the outer surface will be damaged by contact or will acquire a less attractive appearance in some other way. The result of said effects is that it may be the top layer which in the end determines the life span of the solar cell unit, even though, generally speaking, it is not the most expensive part of solar cell unit.
For that reason there is need for a system with which labour intensive cleaning of the solar cell unit can be avoided and, more particularly in the case of plastics based top layers, the life span of the solar cell unit can be extended.
The present invention has for its object to resolve these problems and to this end provides a solar cell unit comprising a back electrode, a photovoltaic layer, a front electrode, and a transparent top layer, characterised in that on top of the top layer at least one removable transparent polymer film is present. When during use the solar cell unit according to the invention gets dirty or is damaged, so that the transparency of its surface is reduced to an unacceptable level, the top polymer film is simply removed, giving a fresh surface which is clean and undamaged. This process can be repeated as many times as there are removable transparent polymer films available.
In the context of the present specification the term top layer refers to a stable top layer, that is, a top layer of which the internal physical and chemical properties, e.g., the degree of polymerisation, are not capable of changing substantially during use.
The number of removable transparent polymer films is determined by the conditions in question. With a too high number of transparent polymer films, the quantity of light reaching the active layers of the solar cell unit through the transparent polymer films becomes too low. When the number of transparent polymer films is too low, the solar cell unit surface cannot be renewed often enough, as a result of which the effect of the invention is insufficiently achieved. Preferably, there are 2-10 removable transparent polymer films stacked one on top of the other on the top layer, more preferably 3-5 films. The number of removable transparent polymer films will also depend on the use to which the solar cell unit is ultimately put. In the case of solar cell units employed in highly polluting surroundings, e.g., alongside motorways, a higher number of removable transparent polymer films will be opted for than when the solar cell units are employed in less polluting surroundings.